Development of a Hybrid Drive that Combines Large Capacity and High-Speed Performance

The hybrid drive, which combines the cost advantage of rotating magnetic disc storage with the high performance of NAND flash memory, has recently been attracting attention from computer users looking for fast, large capacity drives. The combination of a hard disk drive (HDD) and NAND flash memory has the potential to deliver a solid-state drive (SSD) like user experience. Hardware, firmware and architecture design are all essential to creating a product that will meet user expectations in terms of performance, capacity and cost. In the case of the latest technology, for example, that includes developing a new cache algorithm that places frequently used data into NAND flash memory, resulting in a hybrid drive with SSD-like performance.

1. Introduction

Large numbers of video and other types of data are being handled more frequently than ever before. People want to store more data on devices installed in PCs (e.g., HDDs), yet still achieve increased data access performance. For example in operating system (OS) data readout performance for quick booting PCs, as well as improved data read/write performance while PCs are in operation. HDDs are still the major player in the storage device arena and are used in numerous fields, though there are limitations on performance improvement potential. For this reason, SSDs, which feature high-speed data access, are attracting attention and the SSD market is expanding. However, HDDs still have the advantage in cost per gigabyte. Given these circumstances, Hybrid drives, which combine HDDs with NAND memory, have come into their own as storage devices capable of realizing both high capacities and high performance. Now, by adopting an intelligent cache algorithm that learns what data should be stored in the NAND memory cache, the latest generation of these drives is set to deliver HDD capacity and SSD-level performance to both enterprises and consumers.

2. Why consider Hybrid Drives?

As the amount of data created increases in tandem with the popularity of sharing data via the internet, the capacity of storage devices such as the HDDs used in PCs is also increasing. Still, more and more SSDs are being employed in notebook PCs because of the advantages they offer in terms of increased data access speeds, decreased PC start-up times, compact size, and low power consumption.

The Hybrid drive achieves both a high capacity and high performance by combining traditional NAND memory technologies, NAND memory handling technologies (instrumental in SSD technologies), and HDD product development technologies.

Figure 1 Forecast Share of Built-in Storage Products for Notebook PCs. The number of Hybrid drives built-in to notebook PCs is predicted to increase greatly after 2012.

Figure 1 shows a forecast of what types of built-in storage devices will be used in notebook PCs. Full- fledged adoption of hybrid devices, which simultaneously realize both high capacities and high-speed data access performance while keeping bit costs low, is predicted to start in earnest in 2012 and are estimated to account for about 25% of the market in 2015 (1)

3. Architecture

The basic structure of a Hybrid drive is shown in Figure 2. For the cache memory, the HDD uses low-capacity, high- speed DRAM (volatile memory) and a large-capacity magnetic disk (non-volatile memory). NAND memory (non- volatile memory), which has medium capacity, read/write speeds, and bit costs compared to DRAM and magnetic disks, is added as a secondary cache in this HDD.

Figure 2 Basic Configuration of Hybrid Drives – in Hybrid drives, read/write speeds can be improved by making use of 3-tiered storage system consisting of DRAM, NAND memory and a magnetic disk.

(Note 1), (Note 2), and (Note 3): 1 MB represents 106 bytes, 1 GB represents 109 bytes, and 1 TB represents 1012 bytes A newly developed cache algorithm dynamically studies data access patterns and stores frequently accessed data in non-volatile NAND memory. This allows access performance to be improved even if no cache hits occur in DRAM during use, for example when booting the PC. Thus, the algorithm can help the hybrid drive achieve performance levels close to that of SSDs.

PC systems recognize the Hybrid drive just as they do conventional HDDs and SSDs. There is therefore, no need for additional software and existing HDDs or SSDs can be replaced directly with this drive.

4. Hardware Development

In order to keep hardware costs low while achieving high performance, Toshiba has developed a NAND controller for managing data read/write to and from NAND memory. This controller serves as a bridge chip positioned between the System on a Chip (SoC) and NAND chip. The NAND controller and Synchronous DRAM (SDRAM) are connected in parallel to the Double Data Rate 2 (DDR2) SoC interface; different address spaces are allocated to the NAND controller and SDRAM to allow the existing HDD and NAND chips to share the SoC.

To avoid totally redesigning the HDD the afore-mentioned NAND controller and NAND memory need to be mounted onto the HDD circuit board. The conventional four-layer printed circuit board has been changed to a six-layer board allowing the components to be mounted on the same size board as that used by the standard HDD as shown in Figure 3.

Figure 3 Printed Circuit Board Assembly for Hybrid Drives – The NAND memory and NAND controller are mounted on a board the same size as that of HDDs by using small-sized SDRAM and increasing the number of layers in the multilayer board

The amount of space needed to mount the components was further reduced by adopting small sized DDR2 (rather than DDR1) for SDRAM. However, it was impossible to mount more than one NAND memory module. The candidate NAND memory modules and a performance comparison of sequential write operations are shown in Table 1. With respect to NAND memory, single level cell (SLC) technology has a higher cost of capacity per unit than multi level cell (MLC) technology, but also offers much higher performance. If the number of dies (NAND chips) is increased, the drive’s performance will be improved considerably because of parallel processing. Since this Hybrid drive could not include more than one NAND memory, a cost/performance trade-off analysis was performed, which selected the 8-GB SLC (which uses four 2-GB dies).

Table1: Comparison of Sequential Write Performance of NAND Memories

In typical HDDs, random access performance is restricted by seek-times and rotational delays; in most cases, command reordering techniques are used to minimize the impact of these delays, but HDDs are still limited to 200-300 Input/Output per second (IOPS). NAND memory performance is restricted by the access speed to the NAND memory, but this can be improved by increasing the number of channels and the number of dies to enable parallel access. With such techniques, it is possible to enhance the random read/write performance of NAND memory up to several thousand IOPS, an order of magnitude better than that of a standard HDD.

5. Software Development

The capacity of the NAND memory mounted on the Hybrid drive is 8 GB, approximately 1% of the capacity of a 1 TB (Note 3) HDD. Effective use of NAND memory as a cache requires careful consideration of what data to place in the NAND memory because of the limitations imposed by its capacity. The Hybrid drive’s design concept is shown in Figure 4.

Figure 4 Design Concept of Hybrid Drives – Data is controlled so that recently written and read out data is stored in NAND memory, allowing read/write speeds to be improved

Figure 5 Benchmark Scores and Magnetic Disk Read Counts – Data storage locations are optimized during the first iteration, so repeating the benchmark reveals improved read/write speeds, resulting in an increase in score to about three times that of a conventional HDD.

The Hybrid drive consists of the following three storage tiers: SDRAM, NAND memory, and magnetic disk, each of which has different read/write speeds. When the system requests that data be written, the Hybrid drive will write the data to the NAND memory before writing the data to the magnetic disk (data will be written to disk later). When the system requests that data be read from a specific address, the drive will check whether the data for the specified address is stored in NAND memory. If the data is present in NAND memory, the drive will quickly send the data to the system. If not, the drive will read the data from the magnetic disk and send the data to the system, then copy the data to NAND memory. Because of this algorithm, data that was recently written to the Hybrid drive or read out from the drive is contained in NAND memory. If the system requests that data be read from the same address again, the drive can return such data to the system very quickly because the required data is stored in NAND memory.

Using a Hybrid drive prototype with a 16-GB NAND memory a series of six iterations was conducted with the performance-benchmarking tool (PCMark® (Note 4) Vantage HDD Suite). As for the benchmark results, the relationships between benchmark scores and magnetic disk read counts are shown in Figure 5. The Hybrid drive must read data from the magnetic disk if the data is not stored in NAND memory. After the second measurement, the magnetic disk read count decreases dramatically, since most of the data is read out from NAND memory. In the first measurement, the benchmark score is only slightly better than that of a conventional HDD, but the scores for subsequent benchmarks exceed those for the first measurement by a factor of three. These scores are the result of studying the system’s data access patterns and caching the necessary data into NAND memory.

Figure 6 shows an example time comparison in which an HDD, Hybrid drive, and SSD were installed in three otherwise identical PCs. The time to load the same application was then measured. The load time required for the Hybrid device is much less than that of the conventional HDD and is roughly equal to that of the SSD.

Figure 6: Comparison of Application Load Times – On the Hybrid drive, applications started faster that they did on the HDD and nearly as fast as they did on the SSD

Based on these evaluation results, we can confirm the effectiveness of the Hybrid drive concept.

6. Conclusion

The development of a Hybrid drive containing three storage levels comprising SDRAM, NAND memory, and magnetic disk answers the need for many users of fast, high capacity storage. By utilising an algorithm that controls how and where data is stored and cached, SSD-comparable performance can be achieved for repetitive access cycles such as OS access times during PC boot sequences.

To further improve Hybrid drive performance, a specification for standardization of hints, which are data provided by PCs that give hints as to whether to keep data in NAND memory, is now being discussed at the Serial ATA International Organization (SATA-IO), the standard-setting organization for Serial Advanced Technology Attachment (SATA). In the NAND memory field, efforts to reduce costs by integrating components and miniaturizing processes are currently underway.

Toshiba continues to improve the cache algorithm to intelligently exploit the 3-tiered structure of SDRAM, NAND memory, and a magnetic disk, and also to develop second-generation Hybrid drives with even higher performance.

Reference

(1) IDC. WW2012 HDD Market Update. IDC. 2012.

About Toshiba Electronics Europe

Toshiba Electronics Europe (TEE) is the European electronic components business of Toshiba Corporation, which is ranked among the world’s largest semiconductor vendors. TEE offers one of the industry's broadest IC and discrete product lines including high-end memory, microcontrollers, ASICs and ASSPs for automotive, multimedia, industrial, telecoms and networking applications. The company also has a wide range of power semiconductor solutions as well as storage products like HDDs, SSDs, SD Cards and USB sticks.

TEE was formed in 1973 in Neuss, Germany, providing design, manufacturing, marketing and sales and now has headquarters in Düsseldorf, Germany, with branch offices in France, Italy, Spain, Sweden and the United Kingdom. TEE employs approximately 300 people in Europe. Company president is Mr. Takashi Nagasawa.

Toshiba Corporation is a world-leading diversified manufacturer, solutions provider and marketer of advanced electronic and electrical products and systems. Toshiba Group brings innovation and imagination to a wide range of businesses: digital products, including LCD TVs, notebook PCs, retail solutions and MFPs; electronic devices, including semiconductors, storage products and materials; industrial and social infrastructure systems, including power generation systems, smart community solutions, medical systems and escalators & elevators; and home appliances. Toshiba was founded in 1875, and today operates a global network of more than 550 consolidated companies, with 202,000 employees worldwide and annual sales surpassing US$74 billion.